Things necessary for any singing:
breath - when to breathe, how to breathe, when you'll need to breathe
posture - feet shoulder-width apart, shoulders back, head up, hands by sides
support - tightening abdominal muscles, particularly for high notes, loud notes, or when running out of breath
preparing a note - this is difficult to explain, but each note must be thought of a beat before you sing it, and some notes need you to change the part of the voice you're singing in
tone - breathy or strong, head voice or chest voice, bright or dark
mouth shape - up and down rather than wide, soft palette raised (depending on the piece)
staying in tune - self-explanatory
whether you're going to faint - I haven't ever seen someone faint in performance, but I have seen people have to sit down, and if you stand still for too long it is a definite possibility
Things necessary for any piece:
vowels - avoiding dipthongs (e.g. sky is said "skah-ee" with them melded together, but sung "skah-ee" with the ee at the very end), what vowel it is (oo, oh, or, ah, a, au, eh, ee), bright or dark
diction - singing the voiced consonants (e.g. p, d), accenting unvoiced consonants (e.g. s, t)
tempo - are you rushing, are you slowing down, should you be
time signature - where are you in the bar, which beats are strong and which weak
dynamics - loud or soft or in-between, getting louder, getting softer, sharp changes in dynamic
accents - which notes are emphasised
staccato/legato - should notes be strung together smoothly, or each sung separately
cut-offs - when in the bar do you stop singing, maintaining the note until that point
the rhythm - what is the actual rhythm you are supposed to sing
the tune - what is the actual tune you are supposed to sing
the words - what are the words you are singing
what is coming next - in two bars, what will you be singing
the structure of the piece - where are you up to, is this a high point or a low point, are you repeating a section
Things necessary for any group:
the conductor - what are you being told to do, are you doing what is indicated
the ensemble - do you sound like everyone else, do you fit in with their sound, are you in tune and in time
blending - does your tone match theirs, do your vowels match theirs
Things necessary for any performance:
the meaning/emotion of the words - what should you be feeling, what should the audience be feeling
smiling/looking alive - keeping an interested expression on your face (it takes conscious effort for many performers, including me)
not fiddling with anything - don't touch your face, don't shift your feet, don't move your hands, look at the conductor, the ensemble or the audience
moving with the music - feel the music and move with it (this should be natural, but many are trained out of it)
props - sometimes you are holding something in your hands, maybe a candle (particularly for christmas concerts), or an instrument in some choirs, in which you need to think about how to use it
choreographed movement - I object to choralography on principle, but many choirs use it, pieces may also specify clapping, stamping or clicking at certain places, some specify standing and sitting, sometimes you sing while walking
That is actually almost thirty things a chorister should keep in mind, which you would expect to be impossible. However, with practice, tasks which are closely associated get mixed into a single "thing to think about". This can be seen in the difference between learning drivers and experienced ones - the learners have to pay very close attention to everything they do in case they forget something, while for more experienced drivers, driving is one task, so they can divert attention to conversation or music. In this way for a chorister breath, support and tone might all be combined, or vowels, diction, the words and meaning of the words. Posture, fiddling, not fainting, choreographed and natural movement all go together. The better a chorister, the less they have to think about each of these things, so the more they can prioritise. When choristers are very good, most of their attention goes to the conductor and the ensemble - to being totally together, singing exactly what is written, what the conductor wants and what everyone else is singing. It is quite remarkable when a choir achieves this.
Many people are aware of studies which disprove effective multitasking. However, these studies usually do not actually test the effectiveness of common forms of multitasking, only ones that are easy to measure. I strongly hold that multitasking is possible, but only when the tasks are so closely associated they are not thought of as separate, or so automatic they need not be thought of. It is multitasking to walk and talk. It is multitasking to read aloud. It is multitasking to write or type. It is multitasking to have a conversation over dinner, to play a musical instrument, to cook. Very few of us do one thing at a time. To move and observe and think is to multitask. It is not impossible to multitask - it is impossible not to.
